This add-on is not compatible with your version of Firefox because of the following:

Version 0.8.4
Released August 24, 2012
134.1 kB
Works with
Firefox 14.0 and later

* Fix exception when button is not in toolbar* Temporary fix for disappearing of adblock plus button under certain circumstances (see toolbarbutton.js:133)+ Update button after customizing the toolbar (eg. adding the button from palette)